How to calculate j(B) in radiation belt?...

This problem is dedicated to flux calculation in Earth's radiation belt
where the particle motion is adiabatic.

Say we know the proton flux pitch-angle distribution j(alpha). These
particles do bounce motion along field line (McIlwein parameter
L=const). The magnetic field strength (B) in different points of line
is different.

How can we calculate dependence of proton flux j on B if we know
j(alpha) and L=const?

Maybe I had to post this message in another science group to obtain the
more exact answer. Which?